Photosynthetic culture in aqueous liquids is often used for the production of algae. Two main types are known in the art, the open-ended systems such as ponds and basins and the closed systems such as photo-bioreactors (PBR). The aqueous liquid provided in the system typically includes water and the photosynthetic culture organisms such as algae or other species that use photosynthesis, such as microorganisms. Photosynthetic culture is known to require a light intensity much lower than the maximum solar light intensity. Studies have shown that optimal intensities for photosynthesis can be of the order of 10% of the maximum solar light intensity. Light distribution within an aqueous system is often referred to as “light dilution” in the field of algae culture. The higher the light dilution factor, the more significant the impact on the production of algae.
It is well known that the sun moves westward in the sky during daytime and that its maximum altitude (culmination) changes throughout the year. The sun is at its maximum altitude at solar noon each day. Solar noon occurs exactly half way between sunrise and sunset. In the northern hemisphere, the sun is due south at solar noon. The sun altitude passes from about 68° at summer solstice to about 21° at winter solstice for locations close to the 45th parallel of latitude. At these latitudes the maximum solar altitude at both spring and fall equinoxes is about 45°.
The prior art discusses different configurations to allow light to penetrate the aqueous liquid of the aqueous systems. These configurations either do not take into account the solar position throughout the day and throughout the year or can exhibit prohibitive optical losses or alignment precision requirements.
In prior art systems, light distributors are used in open and closed photosynthetic culture aqueous systems to capture the light from the sun and distribute it within the volume of aqueous liquid. In some systems, the light distributor includes two light distribution walls provided at an angle at a bottom end to create a V-shape or provided in a substantially parallel orientation to create a rectangular or parallelogram shape. The light distribution walls are made of a transparent material allowing sunlight to pass therethrough. An interior side of the two light distribution walls creates an elongated channel. The sunlight is received in the elongated channel and is distributed through the light distribution walls in the aqueous liquid. The light distribution walls are disposed in the photosynthetic culture such that at least part of an exterior side of the walls is in contact with the aqueous liquid.
The adjacent light distributors have a longitudinal dimension much greater than the height of their walls, thereby creating elongated channels. In use in a photosynthetic culture aqueous system, a plurality of adjacent light distribution channels are provided at the interface between the air and the aqueous liquid.
Prior art light distributor systems are static, as shown in FIG. 1. FIG. 1 shows the impact of the inclination of the sun on the light dilution in the aqueous liquid for a V-shaped light distribution channel 402 (FIG. 1A, FIG. 1C, FIG. 1E) and for a parallelogram-shaped light distribution channel 404 (FIG. 1B, FIG. 1D, FIG. 1F) in a static light distribution system 400, at three solar noon inclinations including summer (FIG. 1A, FIG. 1B), spring or fall (FIG. 1C, FIG. 1D) and winter (FIG. 1E, FIG. 1F).
A hypothetical light entry surface 412 at the top surface of the V-shaped light distribution channel receives the sun rays. The light distribution surface 414 is along the walls of the V-shaped channel or of the parallelogram channel. The horizon 418 is shown schematically.
The sun rays are illustrated schematically as entering the light distribution channels from the angle at which the sun is located. The aqueous liquid 406 is provided outside of the light distribution channel and is represented by the dotted filling surrounding the channel. The container for the aqueous liquid is omitted to simplify the drawings and the parallelogram channel therefore appears bottomless on the figure. The hatching represents the region of light dilution 408 in the aqueous liquid and varies depending on the orientation and reflection of the sunlight rays within the channel.
When the photosynthetic culture is an open-ended system, the two light distribution walls are sidewalls of a transparent structure and can be joined at the bottom to form a unitary transparent structure which is V-shaped or parallelogram-shaped. A transparent top surface can be provided to close the structure.
For closed systems, such as a PBR, elongated conduits are shaped and juxtaposed to create a light distribution channel or region between the rows of conduits. The elongated conduits are made of a thin transparent material and the aqueous algae culture occurs within the conduits. The elongated conduits can be provided with substantially flat sides. The sides of the adjacent elongated conduits therefore act as the light distribution walls and the solar rays are able to be distributed within the conduits. The conduits may be drop-shaped or rectangular depending on the application.
There is a need to improve light distribution in open-ended and closed aqueous systems to benefit from exposure to as much light as possible from the sun.